Touch control for polyphonic musical instruments



Feb. 17, 1959 E. w. HEROLD 2,873,637

. TOUCH CONTROL FOR POLYPHONIC MUSICAL INSTRUMENTS Filed March 26. 19542 Sheets-Sheet 1 E'yj. ff f6 y Eff/JTAA/CE BY LA L QAM :l TTOR NE Y E.W. HEROLD Feb. 17, 1959 2 Sheets-Sheet 2 Filed March 26, 1954 la m a w.m z m ma M W Il NAM GIS- 1 MW E z z im ,m .l a F m z rw f d f I .n we fa mm lW- w rm z IE.. 4 z z l www...

United States Patent O TOUCH CNTROL FUR POLYPHONIC MUSICAL INSTRUMENTSEdward W. Herold, Princeton, N. J., assignor to Radio 'Corporation ofAmerica, a corporation of Delaware Application March 26, 1954, SerialNo. 419,004

The terminal fteen years of the term of the patent to be granted hasbeen disclaimed 2 Claims. (Cl. S11- 1.04)

This invention relates to electronic musical instruments, and moreparticularly to a new system for controlling the output of suchinstruments.

The use of electronic means for the creation of music is well-known andhas been embodied in numerous commercial instruments, of which the mostsuccessful have been those which improve upon or imitate the sounds oftheir acoustical counterparts.

An ideal electronic musical instrument should be one which is capable ofproducing several notes at a time, i. e. a-polyphonic type. Such anideal musical instrument should minimize the physical effort requiredfor playing and include means for easily controlling the attack,overtone content, or loudness of the tone produced.

In accordance with the present invention, circuits and playing keyarrangements are provided whereby touch alone, without mechanical motionof the key, is suicient to produce a note. In one form, a singlemultigrid tube or a double triode tube is used as a combination tonegenerator and radio frequency oscillator. The tone generator is normallyinoperative when the radio frequency oscillator is operating. Touchingthe key stops the radio frequency oscillator due to body capacitance andlosses, so that the tone generator operates. In other forms of theinvention, motion of the key or the location of the finger on the key isused to control one of the other important factors such as attack,decay, loudness or timbre. Use of conductive rubber and light beamcontrol are embodied in other forms of the invention.

1t is an object of this invention to provide an improved polyphonicdevice.

It is a further object of this invention to provide an improvedpolyphonic device which may 'be operated with a minimum amount ofphysical effort.

It is still a further object of the invention to provide r an improvedpolyphonic device in which the attack, overtone, loudness or other soundcharacteristic may be easily controlled.

Other objects and` advantages of the present invention will becomeapparent and immediately suggest themselves to those skilled in the artto which the invention is directed from a reading of the followingspecication in connection with the accompanying drawing in which:

Figure 1 is a schematic block diagram ofV a touchoperated musical systemembodying the present invennon;

Figure 2 is a detailed schematic showing of a form ofcapacitance-operated control circuit applicable for use in the system ofFigure l;

Figure 3 is schematic showing of a touch control system embodying thepresent invention in which a single tube serves yboth as a controlcircuit and a tone generator;

Figure 4 shows, by way of example, an arrangement for controlling thevolume or other characteristic of a tone in accordance with theinvention; i

2,873,637 Patented Feb. 17, 1959 ICC Figure 4a is a schematic showing ofa circuit which is substantially equivalent to the circuit of Figure 4;y

Figure 5 is schematic showing of an arrangement in accordance with theinvention in which the illustrated touch control makes use of bodyresistance;

Figure 6 is a schematic showing of an arrangement in accordance with theinvention illustrating touch control combined with tone control;

Figure 7 is a schematic showing of an arrangement in 'accordance withthe invention wherein touch control is combined with control of attack;

Figure 8 is a schematic showing of an arrangement in accordance with theinvention wherein a photo-electric device is associated with a controlcircuit; and

Figure 9 is a set of characteristic curves representing variousconditions of operation of the circuit shown in Figure 7.

In the conventional piano or organ, it is necessary not only to placethe iingers on the proper key, Ibut to actually press these keys and doan appreciable amount of Work. With an electronic instrument, such asone embodying one form of the present invention, it is not necessary topress a key because the mere touch of a finger sulhces to sound thenote. 'Referring particularly to Figure l, an arrangement of keys 12 isshown, connected to a set of capacitance-operated control circuits 14.The control circuits, in turn, actuate tone generators 16. When the ngerof an operator is placed in contact with, or very close to, a particularkey, the capacitance-operated control circuit operates a tone generatorproducing a desired note in a utilization device, such as will include aloudspeaker 17.

One form of capacitance-operated control circuit has been describedheretofore in an article by F. H. Shepard, proceeding of the I. R. E.,December, 1936. When the control grid connection of a vacuum tubeoscillator, using a grid-leak Vbias circuit, is touched with the hand,the oscillations stop. There are two reasons for this. First, theadditional grid-to-ground capacitance, combined with the grid condensercapacitance, form a voltage divider which reduces the grid excitation;second, the radiation and ohmic loss of the body load the oscillatorcircuit severely. If, instead of touching the grid connection, the hotend of the grid inductance is touched, only the second cause isoperative and the phenomenon is somewhat less sensitive, although itstill works. Since, in the usual grid-leak biased oscillator, the platecurrent increases when the oscillation is stopped, one is able to usethis principle for a simple control circuit.

Of course, although this invention is described in connection withvacuum tubes, it is evident that various other types of devices may Ibeused. Transistors, for example, are coming into extensive use. Suchtransistors are small, compact and require no filament power. The samegeneral principle, embodying the present invention and described inconnection with vacuum tubes, may be employed with transistors. Thecircuits, however, would be altered to conform to the difference inbehavior between the transistor and the electron tube.

Referring particularly to Figure 2, there is shown in detail a diagramof one form of capacitance-operated control circuit, such as might beused for one of the blocks of Figure 1. As shown in Figure 2, thecircuit involves only a single note of an electronic musical instrument,it being obvious that a plurality of circuits of the same type may beemployed to produce more than one note without departing from the scopeof this invention. A triode oscillator tube 1S uses a Hartley typeoscillator circuit comprising an inductance 20 and a capacitance 22. Aresistor 24 provides a grid leak bias for the tube. A connection 26, tothe grid 28, provides means to permit an operator to affect theoperation of the oscillator.

The bias, which is generated across the resistor 24 when tube 28 is inan oscillating state, is applied to the control grid 36 of an amplifierpentode electron tube 38, by means of the resistance-'capacitance filtershown as 32 and 34 in Figure 2, together with the grid resistor 40. Thispentode amplifier 38 is used in the path between the electrical tonegenerator 42 and a common audio amplifier which is coupled to thepentode amplifier through a capacitor 44 and a loudspeaker for all thenotes of the instrument (not shown). The operating voltages for thepentode 38 are chosen so that the bias supplied by the triode tube cutsofi the anode current of the pentode, and no tone signal is transmittedto the common audio amplifier. The circuitry associated with the tube 36and the common amplifier and loudspeakers provides for the utilizationof the generated tones.

When the connection 26 is touched with the hand, or when the hand isbrought sufficiently close to it `(the choice as to how close the handshould be brought to stop the oscillator is governed by the constants ofthe oscillating circuit and, in particular, the size of the gridcapacitor the oscillator stops oscillating and the bias across thegrid-leak, resistor 24 is thereby removed'. The pentode 38 is then nolonger cut off and operates as a normal amplifier with its bias suppliedby cathode resistor 46. Thus, as soon as the control element orconnection 26 is touched, the tone generator signal is transmitted tothe common audio amplifier and can be heard in a loudspeaker.

A pentode amplifier is shown in Figure 2 because it is essential that,when full cut-off bias is appliedV to its grid, no tone must get intothe plate circuit by the way of inter-electrode capacitance. Toeliminate shock hazard in case of a shorted tube, a capacitor may beinserted in the lead to the key, and the key grounded by means of an R.F. choke, provided such a capacitor is substantially larger than thecapacitor 30. This will not affect operation in any way.

The use of two tubes for each capacitance-operated control circuit, inaddition to the tube used for the tone generator, all duplicated foreach note, represent a large number of tubes. For this reason, there isprovided in this invention, improved systems involving the useof fewertubes.V However, it is still recognized that the principles involved inthis invention are notV necessarily limited to vacuum tubes.

Referring particularly to Figure 3` of the drawing, a touch controlsystem embodying the present invention using a single tube is shown inthe form of a double triode. This single tube serves as both controlcircuit and tone generator. Two triode sections 46 and 48 are connectedin an audio-frequency, multi-vibrator type of circuit. In addition, inthe grid-cathode circuit of the sect-ion 46 there is connected aradio-frequency Hartley type oscillator circuit comprising aninductancef and a capacitor 52. A grid leak resistor 54 and a capacitor56 are placed onthe ground end of the coil. A capacitor 53 between thecathode 6ft and a tap 62 on the radio frequency coil is aradio-frequency by-pass capacitor which does not appreciably passaudiofrequency. Similarly, a capacitor 6,4f between the plate 65 and groundis a radio-frequency by-pass capacitor which affects slightly the 'audiofrequency generated by the multivibrator but not its oscillatingcapability. A resistor 68 and a capacitor 70 form a radio-frequencyfilter circuit to conduct the oscillator grid-leakyoltage, producedacross the resistor 54, onto the grid 72 of the second triode through aresistor 73. VThe multi-vibrator circuit consists of a common cathoderesistor 74 to ground, the plateresistors` 76 and 7S. in the two triodeplate circuits, the plate-tofgrid coupling capacitor 80 between thefirst andAV se'condtriodesilam a V ':apacitor S2 from the plate 84 Vtoground. `Wl1n usedby itself, this type of multivibrator circuit is knownas Potter circuit (because it was described by Potter in the EProceedingby the I. R. E. in I une 1938). When the control electrode 86 is touchedby an operator, the radio-frequency oscillator of the rst triode 46stops oscillating. The bias is thereby removed from the second triode 48so as to actuate the audio multi-vibrator circuit, thereby producing anaudio tone rich in harmonics, as desired by a musical instrument. Theradio frequency oscillator therefore acts as a control circuitassociated with manual means for varying its operation.

Referring now to Figure 5, a different touch control circuit is shown,which makes use of body resistance. A tone generator liti having anelectrical audio signal output is coupled to the grid 114 of anamplifier tube 112 through a capacitor liS. A source of negativepotential designated as E- is applied to the grid through resistors 116'and 118. An output voltage is developed across the plate 12) and cathode121. The associated output circuit is not shown for purposes ofsimplification. In this embodiment of the invention, the finiteresistance of the human finger to electrical current is utilized. Atouch'control electrode 22, which may, for example, be equivalent to akey on a piano or organ, comprises two closely spaced parallel bars H4and 126. When the finger is touched to these bars, the finger resistancecornpletes the circuit to ground. If the resistance of the resistorld'in series with the cut-off voltage is very high compared to thefinger resistance, the bias on the audio amplifier i12 willv be reducedsubstantially. The amplifier then becomes conducting and the audio tonefrom the tone generator is amplified. A capacitor 125 or disturbance isdetectable'to the performer.

In the embodiments of the invention described up to this point,reduction in physical effort in playing polyphonic instrument by use ofa touch-control system has been indicated. However, in many cases, it isimportant that the performer be also able to control either theloudness, the attack or the harmonic content (timbre) of the note, as hewishes. Y

A simple circuit in which touch control may be use to control volume orother sound characteristics is shown in Figure 4. It is intended thatany one of the previous circuits can be used to initiate control, i. e.by touch only, whereas by additional control of finger position, asecond auxiliary sound characteristic can be altered. A series ofcontact bars 88 is provided for each note, so

Y that, when the finger is placed on the proper pair of contacts, thecircuit is completed through the nger resistance and appropriateauxiliary control thereby attained. The finger may also be slid forwardand backwards, that is, from left to right as viewed in Figure 4, inorder to obtain a continuously variable control, while theperformer isplaying the particular note to which the contact is connected. Figure 4aillustrates an equivalent circuit. Various resistors and 92 may beincluded in an electrical circuit by movement of the finger. Appro`priate circuits to which such means may be applied are not shown. Y

Referring particularly to Figure 6, another method of using a singletube is shown and combines touch control with a tone controlV circuit'lnthis embodiment, a radio frequency oscillator and an audio oscillatorare combined within a single tube envelope.

A pentagrid tube V12S, such as is commonly used in broadcast receiversfor the converter stage, is employed in this embodiment. A radiofrequency oscillator provides a capacitance or touch-operated controlcirfcuitand the audio oscillator Yprovides a musical tone.- Only one ofthese oscillators is operative at any*giventime. Y The cathode and thegrid 132 are connected as a Hartley type radio-frequency oscillator, forwhich the grid 134 acts as an anode. The tuned circuit comprises aninductor 133 and a capacitor 135. The grid 154 is by-passed to groundthrough a radio-frequency by-pass capacitor 136. The suppressor grid 151is directly connected to the cathode. A key 138 comprises a metal spring140 and a bottom metal plate 142 separated by a dielectric material 144.Grid leak capacitance is provided by a capacitor 137. The bias acrossthe grid leak 146 is applied to the second control grid 134 throughresistor-capacitor filter 150 and 157 and through resistor 152. Whenradio-frequency oscillations are present, this bias is enough to cut offthe second control grid 134.

When the key 138 is touched the radio-frequency oscillator stopsoscillating and the bias across the grid leak resistor 146 is removed.This, in turn, removes the bias from the second control grid 134 andpermits this grid to operate on the electron current available to itthrough resistors 150 and 152. The screen grids 154 and 148, which areinternally connected in the tube and the-second control grid 134 form anegative-transconductance audio oscillator, which oscillates at an audiofrequency determined by the time constant of the combination comprisingthe resistors 155 and 152 and the capacitors 136 and 156. Such anoscillator, in its simple uncombined form is well-known and Wasdescribed in an article by the applicant on negative resistance devicesin the Proceedings of the I. R. E. in October 1935. Thenegativetransconductance oscillator cannot oscillate however, if thebias on the grid 148 is in excess of the cut-off bias of that grid sothat the audio tone is generated only when the key 138 is touched. Inthe event that the grid 134 is a remote cut-ofi` grid, as is often thecase in commercially available tubes, one must only provide sufficientbias to cut-off the grid to prevent oscillation, since the smallnegative transconductance at high control -bias voltages is notsufficient for oscillation. It is therefore seen that when theradio-frequency section of the tube is oscillating, thenegative-transconductance audio oscillator cannot oscillate. No audiotone is then produced in the plate circuit of the pentagrid tube, whichmay be connected to an audio amplifier or other utilization circuitthrough a coupling capacitor 158.

If the finger of an operator touches the spring material member 140 ofthe key 138, the radio-frequency oscillator becomes inoperative. Thebias is therefore removed from the grid 154, and thenegative-transconductance oscillator starts oscillating at anaudiofrequency rate. The audio frequency in the plate circuit may thenbe transmitted to an audio amplifier. It will be appreciated that if thegrid leak 146 has large impedances at audio frequencies, touching thefinger to the metal spring may produce some pickup or hum voltage whichmay be transduced into the audio amplifier in addition to the desirednote. However, this may be overcome by expedients wellknown in the art.

It is seen that, when the finger of the operator is depressed, so as tocurve the metal piece 140 down over the bottom metal plate 142,additional capacitance is introduced between the touched piece 140,which is at audio-frequency ground potential, and the other piece 142 ofthe metal spring, which is connected to the audio output anode 160. Theamount of this capacitance is controlled by the finger pressing themetal spring and acts as a tone control for the audio frequency which ispassed on to the audio amplifier. Thus, when the finger first touchesthe metal spring, the audio tone starts and may be rich in harmony andovertone. lf, however, the finger is depressed harder, the capacitancebetween the metal spring 138 and the bottom plate 142 is increased so asto bypass more and more of the higher frequency from the anode circuit,thereby making the tone more mellow. The performer, therefore, is ableto control the characteristic of the tone produced by pressing the keyharder or less hard. It should be realized that many other ways may beemployed to accomplish this tone control without departing from thescope of this invention. Essentially a variable capacitance may be usedin any type of circuit which will operate with a variable capacitance,whether as a tone control, as a loudness control, or as an attackcontrol. If desired, the spring member may be made of two insulatedpieces, one above the other. The upper piece could be used in the radiofrequency circuit and would be touched by the finger, whereas the lowerpiece would be separately connected to the capacitance circuit. Such anarrangement would minimize the effect of hum in the audio outputcircuit.

It should be appreciated that the oscillator type of capacitive touchcontrol does not require the finger actually to touch a piece of metal.The piece of metal may have a plastic coating, since it is only thefinger capacitance and body loss at radio frequencies which cause theradio frequency oscillator to stop and allow the audio oscillator tostart.

Referring particularly to Figures 7 and 9 of the drawing, a circuit isshown wherein the attack may be controlled by the performer. Control ofattack of an audio tone is considered by many to be more important thana control of volume or tone quality. However, it is realized that thecircuit shown may, with slight modification, be also used to control thevolume, tone content or other sound characteristic.

A pair of triodes 162 and 164, which may be incorporated into a singleenvelope if desired, include a radio frequency oscillator in the firsttriode 162 and an audio oscillator in association with the second triode164. The radio frequency oscillator comprises a conventional Hartleytype having a tapped inductor coil 166 and a capacitor 168 providing thetuned circuit. Resistor 170 and a capacitor 172 provide grid-leak biasfor the oscillator. The radio-frequency oscillations are stopped by aperformer placing a finger on the key 174, as previously described.

The audio oscillator comprises a triode 164 used in conjunction with aresistance-tuned type audio oscillator comprising capacitors 176, 178and 180 together with resistors 182, 184 and 186. This oscillatingcircuit operates only when the finger of the operator touches the keysince the cut-off bias is otherwise impressed on the grid 188 throughthe resistors 190 and 186. When the finger touches the key, theradio-frequency oscillator stops. The bias is therefore removed from'theaudio oscillator, which then becomes operative. At the junc- .tion ofcapacitors 176 and 178, the audio frequency voltage is taken off andapplied to a grid 192 of an amplifier tube 194. It will be noted in thiscase that the grid-leak resistor of the tube 194 is resistor 182. Thelower end of the resistor 182 is connected so that, if the key is notdepressed, a bias designated by a battery 196 is placed on the grid 192of the amplifier tube 194. This bias may be adjusted so that a very weakaudio tone appears and, when the instrument is played very softly, thekey need not be depressed but is merely touched. If, however, the key isdepressed at the end 198 of the key so as to touch the resistor 200 atits lefthand end, it will be noted that this connects the end of thecapacitor 202 which is connected to terminal 19S, to ground through thekey and radio frequency coil, which is very low in impedance at D. C. oraudio frequencies. As a result, the bias voltage is instantly removeduntil the capacitor 202 charges up. The audio tone reaches full volumesuddenly and then gradually decays as in the tone of a piano or banjotype.

Referring particularly to Figure 9 of the drawing, the curves indicatethe type of attack which may be expected from touching the finger to thekey at different places. Each curve shows the amplitude of the audiooutput as a function of time. Curve 204 represents the output 7 whenthekey is touched, but not pressed into contact with resistor 200. Curve206 represents the output when the key is pressed at the end i923. Curve268 represents the output when the key is pressed between the points 198and the middle of the key represented by a tap 199. Curve 21@ representsthe output when the key is pressed at the tap 199, as illustrated in thedrawing. Curve 212 represents the output when the key is pressed betweenthe middle and the end 2l4 of the key. Figure 216 represents the outputwhen the key is played near the end 214. ,A

. The various combinations, such as described, may be obtainedV byjudicious choice of the tapped resistor 20) and the capacitors 2tl2 and139', together vwith the resistor 182 and the capacitors 176 and T178associated with it. The Operation of the circuit can be readilydetermined by examining the curves of Fig. 9 showing the transientcharacteristics of the key circuit.

In connection with this circuit, the key is made of rubber having anelectrically conductive material added. Experiments have shown that theresistance between two points varies with pressureA or tension. Thisconductive rubber has advantages over other types of pressure controlledresistance's, since it is low in cost and practical to apply. Y

Referring particularly to Figure 8 of the drawing, there is shown aphoto-electric cell 22). A light source 222 striking the cathode of thedevice causes current to flow, thereby developing voltage across aresistor 224. Variations in light will produce a voltage variationacross the resistor 224. This variation may be used to vary the gain ofan amplier tube 225. A tone generator 228 is applied tothe grid 232 ofthe ampliiier through a coupling capacitor 230. A resist'or'234 is usedto provide means for biasing the amplifier. A resistor 236 provides a'load for the amplifier.

,When the finger of a performer is placed near one end ofthe slit,almost the full amount of light is put on the phototube. As the ringeris pushed inward, and touches more and more closely the inner portionsof the slit towards the other end, less'and less light strikes thephototube. Thus, the phototube current is a function of the fingerposition and may 'be used to control the amplitude of othercharacteristic of an audio tone. Other' arrangements may include theVlight source within the instrument. The light beam may then be projectedupward and' diffusedV byithe nger. Again, the ingers position woulddetermine the amount of light reaching aereas? the phototube, thusproviding means to vary the amount of current and the volume orothercharacteristic of a tone. Y Y

rIt is apparent that principles described Yin connection with theembodiments shown are not limited to any particular musical instrumentor tone.

The use of transistors in touch control generally, of the typedescribed, can be achievedpby making use of the same general principlesdescribed in connection with electron tubes. i Y

What is claimed is:

l. In a polyphonic musical instrument, theV combination comprising atone generator having an electrical audio-frequency signal output, autilization circuit, means for applying said audio signal output to saidutilization circuit, a radio-frequency oscillator circuit to develop abias, means for applying said Vbias to maintairi's'aid tone generatornormally inoperative, a control circuit for varying the operation ofsaid radio frequency oscil-V l`atcr,`rneans connecting said controlcircuit to said oscillator, means associated with said control circuithaving a'yariable electrical characteristic, said la'st named meansbeing accessible to the linger of an operator, whereby a touch of thelinger of an operator is effective to vary said electricalcharacteristic to render said radio frequency osciilator inoperativethereby reducing said bias to permit operation of said tone generator,and means for varying said audio signal output from said tone generatorYto said utilization circuit.

2. The invention as defined in claim 1 wherein saidy means for varyingsaid audio signal output voltage com# prise electrically conductiverubber member and a resistance associated with said tone generatordisposed in close proximity lto said rubber'xnember whereby a pressureapplied by the finger of'an operator `to said rubber is etectivetoelectrically connect said rubber to a certain portion of lsaidresistance,

References Cited in the ile of this patent UNlTED STATES PA'ENTSr1,661,058 Theremin Feb. 28', 1928 1,847,1l9 Lertes ot al. Mar. l, 19322,l4l,231 Trautwein Dec. 27, 1938 2,142,580 Williams Jan. 3, 19392,20l,232 l-lelberger May 21, 1.940 2,270,789 SmithV Ian.. 20, 19422,732,545 Passow et al Jan. 24, 1956

